FIG. 6 shows the structure of a conventional discharge lamp, disclosed in Japanese Laid Open Patent No. 2005-142071.
This discharge lamp 1 comprises a bulb which is made of quartz glass, and has an arc tube 2 and sealed tube portions 3, and a cathode 4 and an anode 5. The cathode 4 and the anode 5 are provided so as to face each other in an arc tube 2 and are supported by respective electrode rods 6 made from tungsten. Moreover, support tube bodies 7 are fixed in the sealed tubes 3, respectively. Each of the support tube bodies 7 is cylindrical and is made from quartz glass and has a through hole therein, extending in the axis direction. While the electrode rods 6 are inserted in the respective support tube bodies 7 so that the rods are supported by the respective support tube bodies, the rods are attached in a way of sealing to the sealed tubes 3 by connection glass members 8. These electrode rods 6 extend outward from outer ends of the bulb, respectively, and serve as external lead rods from which electric power is supplied to the cathode 4 and the anode 5.
FIG. 7 shows the cathode 4 of the discharge lamp 1 which has such a structure. The cathode 4 is integrally made up of a cylindrical body portion 21, a tip portion 22 having a circular cone shape, and a step portion 23 having an outer circumference which is formed in the shape of steps between the body portion 21 and the tip portion 22. Moreover, the cathode 4 is made of tungsten in which thorium oxide is doped, and a layer of tungsten carbide is formed on the surface thereof. Since the cathode 4 becomes high temperature during lighting of the discharge lamp 1, thorium oxide is returned in the tungsten carbide layer, thereby becoming thorium, and the thorium stimulates electron emission from the cathode 4, so that the luminescent spot of an arc is stabilized. The temperature of the cathode 4 shown in FIG. 7 rises, when light of an arc is irradiated to a face 24 perpendicular to the central axis L of the step portion 23. Therefore, also in the step portion 23, a thorium oxide can be returned, so that thorium can be supplied, thereby increasing the quantity of thorium which can be used.
However, since elements of a digital projector are smaller in size, than those of the conventional film projector, the use efficiency of light decreases. Therefore, the discharge lamp 1 having high intensity is demanded as a light source. If an electric power input is increased to raise the intensity of the discharge lamp 1, expansion and contraction of the arc will occur. The intensity is related to the temperature of the tip of the cathode, in that the temperature of the tip portion 22 becomes high, as the intensity becomes high. Since the speed of evaporation of thorium will become high when the temperature of the tip portion 22 becomes high, the balance of demand and supply of thorium is disrupted. Therefore, the thorium is excessively consumed, so that thorium becomes insufficient, whereby expansion and contraction of an arc occurs. If expansion and contraction of an arc occurs, since an arc shakes, the luminescent spot moves. Since the position of the luminescent spot is not settled at a focal point within a condensing mirror if the luminescent spot moves so that an optical output fluctuates, flickering is generated when light is irradiated on a screen. Moreover, in order that expansion and contraction of an arc may not occur, it is conceivable that the cathode 4 and the anode 5 are arranged so that the distance therebetween may become long, thereby decreasing the temperature of the tip portion 22, or that the diameter at the tip of the tip portion 22 is enlarged, so that heat is dispersed, thereby making temperature per unit area low. However, if the distance of the cathode 4 and the anode 5 becomes long or the diameter at the tip of the tip portion 22 is enlarged, although expansion and contraction of an arc does not occur, since the light intensity decreases greatly, the light intensity of a light source may not be increased.